a) Field of the Invention
This invention relates to an electrophoretic dispersion that comprises charged pigment particles dispersed in a fluorinated dielectric solvent and a charge controlling agent. The dispersion may be used in all types of electrophoretic displays including transmissive, reflective and transflective displays which may have the traditional up/down switching mode, the in-plane switching mode or the dual switching mode.
b) Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles suspended in a colored dielectric solvent. This general type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between the electrodes. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate are required for the active type EPDs. An electrophoretic fluid composed of a colored dielectric solvent and charged pigment particles dispersed therein is enclosed between the two electrodes.
When a voltage difference is imposed between the two electrodes, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, can be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages.
To view a reflective EPD, an external light source is needed. For applications to be viewed in the dark, either a backlight system or a front pilot light system may be used. A transflective EPD equipped with a backlight system is typically preferred over a reflective EPD with a front pilot light because of cosmetic and light management reasons. However, the presence of light scattering particles in traditional EPD cells greatly reduces the efficiency of the backlight system. A high contrast ratio in both bright and dark environments therefore is difficult to achieve for traditional EPDs.
A transmissive EPD is disclosed in U.S. Pat. No. 6,184,856 in which a backlight, color filters and substrates with two transparent electrodes are used. The electrophoretic cells serve as a light valve. In the collected state, the particles are positioned to minimize the coverage of the horizontal area of the cell and allow the backlight to pass through the cell. In the distributed state, the particles are positioned to cover the horizontal area of the pixel and scatter or absorb the backlight. However, the backlight and color filter used in this device consume a great deal of power and therefore are not desirable for hand-held devices such as PDAs (personal digital assistants) and e-books.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD (M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)) and the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026). However, both types have their own problems as noted below.
In the partition-type EPD, there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of the particles such as sedimentation. However, difficulties are encountered and they include formation of the partitions, filling the display with an electrophoretic fluid, enclosing the fluid in the display and keeping the fluids of different colors separated from each other.
The microencapsulated EPD has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a dispersion of charged pigment particles that visually contrast with the dielectric solvent. The microcapsules are typically prepared in an aqueous solution, and to achieve a useful contrast ratio, their mean particle size is relatively large (50–150 microns). The large microcapsule size results in poor scratch resistance and a slow response time for a given voltage because a large gap between the two opposite electrodes is required for large capsules. Also, the hydrophilic shell of microcapsules prepared in an aqueous solution typically results in sensitivity to high moisture and temperature conditions. If the microcapsules are embedded in a large quantity of a polymer matrix to obviate these shortcomings, the use of the matrix results in an even slower response time and/or a lower contrast ratio. To improve the switching rate, a charge-controlling agent (CCA) is often needed in this type of EPDs. However, the microencapsulation process in an aqueous solution imposes a limitation on the type of CCAs that can be used. Other drawbacks associated with the microcapsule system include poor resolution and poor addressability for color applications.
An improved EPD technology was recently disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170 published on Sep. 13, 2001), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001 (corresponding to WO02/56097), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD comprises isolated cells formed from microcups of well-defined shape, size and aspect ratio and filled with charged pigment particles dispersed in a dielectric solvent. The filled cells are individually sealed with a polymeric sealing layer, preferably formed from a composition comprising a thermoset or thermoplastic precursor.
The microcup structure enables a format flexible and efficient roll-to-roll continuous manufacturing process for the EPDs. The displays can be prepared on a continuous web of a conductor film such as ITO/PET by, for example, (1) coating a radiation curable composition onto the ITO/PET film, (2) making the microcup structure by a microembossing or photolithographic method, (3) filling the microcups with an electrophoretic fluid and sealing the microcups, (4) laminating the sealed microcups with the other conductor film and (5) slicing and cutting the display into a desirable size or format for assembling.
One advantage of this EPD design is that the microcup wall is in fact a built-in spacer to keep the top and bottom substrates apart at a fixed distance. The mechanical properties and structural integrity of this type of displays are significantly better than any prior art displays including those manufactured by using spacer particles. In addition, displays involving microcups have desirable mechanical properties including reliable display performance when the display is bent, rolled or under compression pressure from, for example, a touch screen application. The use of the microcup technology also eliminates the need of an edge seal adhesive which would limit and predefine the size of the display panel and confine the display fluid inside a predefined area. The display fluid within a conventional display prepared by the edge sealing adhesive method will leak out completely if the display is cut in any way or if a hole is drilled through the display. The damaged display will be no longer functional. In contrast, the display fluid within the display prepared by the microcup technology is enclosed and isolated in each cell. The microcup display may be cut into almost any dimensions without the risk of damaging the display performance due to the loss of display fluid in the active areas. In other words, the microcup structure enables a format flexible display manufacturing process, wherein the process produces a continuous output of displays in a large sheet format which can be cut into any desired sizes. The isolated microcup or cell structure is particularly important when cells are filled with fluids of different specific properties, such as colors and switching rates. Without the microcup structure, it will be very difficult to prevent the fluids in adjacent areas from intermixing or being subject to cross-talk during operation.
For applications to be viewed in dark environments, the microcup structure effectively allows the backlight to reach the viewer through the microcup walls. Unlike traditional EPDs, even a low intensity backlight is sufficient for users to view in the dark the transflective EPDs based on the microcup technology. A dyed or pigmented microcup wall may be used to enhance the contrast ratio and optimize the intensity of backlight transmitted through the microcup EPDs. A photocell sensor to modulate the backlight intensity might also be used to further reduce the power consumption of such EPDs.
The microcup EPDs may have the traditional up/down switching mode, the in-plane switching mode or the dual switching mode. In the display having the traditional up/down switching mode or the dual switching mode, there are a top transparent electrode plate, a bottom electrode plate and a plurality of isolated cells enclosed between the two electrode plates. In the display having the in-plane switching mode, the cells are sandwiched between a top transparent insulator layer and a bottom electrode plate.
For all types of the EPDs, the dispersion contained within the individual cells of the display is undoubtedly one of the most crucial parts of the device. The dispersion, as stated earlier, usually is composed of pigment particles dispersed in a dielectric solvent. The composition of the dispersion determines, to a large extent, the lifetime, contrast ratio, switching rate, response waveform and bistability of the device. In an ideal dispersion, the pigment particles remain separate and do not agglomerate under all operating conditions. Furthermore, all components in the dispersion must be chemically stable and compatible not only with each other but also with the other materials present in an EPD, such as the electrodes and sealing materials.
The pigment particles in the dispersion may exhibit a native charge, or may acquire a charge when suspended in the dielectric solvent or may be charged using a charge controlling agent (CCA). The use of a CCA in a dispersion with a hydrocarbon solvent such as alkanes and alkylbenzenes is well known in the prior art. The mechanisms of electrically charging particles in a nonaqueous liquid have been reviewed by Fowkes, et al. in ACS Symp. # 200, “Colloids and Surfaces in Reprographic Technologies”, pp. 307 (1982) and by Schmidt, et al., “Liquid Toner Technology”, Chapter 6 in “Handbook of Imaging Materials”, (1991). Particles dispersed in a hydrocarbon solvent may be charged by the addition of a surfactant or CCA. Acid-base chemistry between the particles and the ionic surfactant micelles is believed to result in charging of the particles. The formation of negatively charged particles is enhanced by proton or cation exchange from the particles to the micelles and formation of positively charged particles is enhanced by proton or cation exchange from the micelles to the particles. The magnitude of the zeta potential increases with stronger acid base interaction and decreases with an increasing dielectric constant of the solvent. A diffuse double layer with zeta potentials exceeding 100 mV has been demonstrated in the case of strong acid-base interactions. Examples of typical CCAs for the hydrocarbon dielectric solvents include metal dialkylsulfosuccinate, metal petronate, metal dialkylnaphthalene sulfonate, metal alkylsalicylate, metal alkylaryl sulfonate, metal stearate, Fluorad® perfluoro surfactants from 3M, copolymers of long-chain methacrylate or alphaolefins with acidic or basic comonomers, polyisobutylene succinimides, soy lecithin, N-vinyl pyrrolidone copolymers and the like.
Highly fluorinated polymers having chains longer than C8 have been disclosed in U.S. Pat. No. 4,285,801 (1981) as dispersants or CCAs for EPD applications particularly for EPDs using a hydrocarbon as the dielectric solvent. The polymers include highly fluorinated long chain alkyl or akylaryl carboxylic acids, sulfonic acids, and phosphoric acids, their esters and metal salts, highly fluorinated long-chain alkyl and alkylaryl alcohols, highly fluorinated A-B block copolymers of a long-chain alkyl or alkylaryl alcohol with ethylene glycol or propylene glycol, highly fluorinated poly(alkyl methacrylate) and their copolymers.
Halogenated solvents of high specific gravity have been widely used in EPD applications particularly in those involving an inorganic pigment, such as TiO2, as the charged whitening or coloring particle. The halogenated solvents of high specific gravity are very useful in reducing the rate of sedimentation of the pigment particles in the solvent. Fluorinated solvents are among the most preferred because they are chemically stable and environmentally friendly. However, most CCAs and dispersants suitable for use in hydrocarbon solvents are not effective for dispersions in fluorinated solvents particularly high boiling-point perfluorinated solvents. This could be due to poor solubility or charge separation of the CCAs in these solvents. As a result, pigment particles are very difficult to disperse in perfluorinated solvents. Therefore, EPDs based on perfluorinated dielectric solvents typically show poor stability and switching performance.
To improve the stability and display performance of EPDs based on fluorinated solvents, an electrophoretic fluid comprising 22.5 to 44.25 wt % of a hydrocarbon solvent, 54.42–75.20 wt % of at least one chlorine-free fluorinated solvent and 0.1 to 1.5 wt % of a fluorosurfactant was taught in U.S. Pat. No. 5,573,711 (1996). The presence of a hydrocarbon solvent such as phenylxylylethane, phenyloctane, decahydronaphthalene or xylene was claimed to result in a stronger solvent system that gives a better display performance. However, the use of hydrocarbon solvents in any significant amount is undesirable because it lowers the specific gravity of the solvent and, as a result, increases the sedimentation rate of the pigment particles particularly when a pigment of high specific gravity, such as TiO2, is used.
Thus, there is still a need for an EPD with improved performance resulting from improved design in the dielectric solvent, CCA and particle system selection.